Arrangement for the stereophonic reproduction of signals



R. SCHUTTE April 30, 1963 ARRANGEMENT FOR THE STEREOPHONIC REPRODUCTION OF SIGNALS Filed March 21, 1960 DETECTING CIRCUIT MIXER/ OSCILLATOR FIG. 1

INVENTOR RUDOLF SCHUTTE AGENT United States Patent 3,087,994 ARRANGEMENT FUR THE STEREOPHONIC REPRODUCTION OF SIGNALS Rudolf Schutte, Eindhoven, Netherlands, assignor to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Mar. 21, 1960, Ser. No. 16,214 Claims priority, application Netherlands Mar. 23, 1959 14 Claims. (Cl. 17915) This invention relates to an arrangement for the sterophonic reproduction of signals which are supplied to the input circuit of this arrangement in the form of the sum signal A=+B of the coherent stereophonic signals A and B and in the form of the difference signal AB of these coherent stereophonic signals A and B, which difference signal modulates a sub-carrier frequency in amplitude, whilst the stereophonic reproducing arrangement is also provided with reproducing devices which are situated in separate channels and are fed with the coherent stereophonic signals A and B. The arrangement described can be used in particular in a receiver for stereophonic broadcast reception, the sum signal A-l-B and the difference signal AB, which modulates the sub-carrier frequency in amplitude, modulating the broadcast carrier frequency in amplitude or in frequency, and also in magnetic tape reproducing apparatus, gramophones and the like.

In order to obtain a stereophonic reproduction, it has already been suggested to supply, by means of separating filters, the sum signal A+B to a first channel and the difference signal A-B, which modulates the sub-carrier frequency in amplitude, to a second channel which includes an amplitude detection device for detecting the difference signal AB modulating the sub-carrier frequency, which difference signal is supplied together with the sum signal A-l-B to the separate reproducing devices through an addition and subtraction device.

It is an object of the present invention to provide another design of an arrangement of the type described, which, while improving the quality of the stereophonic reproduction, is distinguished by its simplicity and overmore enables existing arrangements for non-stereophonic reproduction to be completed for stereophonic reproduction with the aid of a minimum of additional elements.

The stereophonic reproducing arrangement in accordance With the invention is characterized in that the signals appearing at the input circuit of this arrangement, which comprise the audio-frequency sum signal A +B and the difference signal A-B which modulates the sub-carrier frequency in amplitude, are supplied together to two amplitude detection devices of oposite conductivity with respect to the audio-frequency sum signal A+B, the output circuit of each amplitude detection device being connected to the input of one of the separate channels.

In order that the invention may readily be carried out, two embodiments thereof will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 is a circuit diagram of a stereophonic broadcast receiver provided with an arrangement in accordance with the invention, and

FIG. 2 is a circuit diagram of a transistorized stereophonic receiver.

The stereophonic receiver shown in FIG. 1 is designed for the reception of stereophonic signals which are transmitted as frequency-modulated signals modulating the same carrier frequency and comprise the sum signal A+B of the coherent stereophonic signals A and B, which may lie in the band of from 30 c./s. to 15,000 c./s., and a subcarrier frequency of 35 kc./s. which is amplitude modulated by the difference signal A-B, the modulation signal produced, which lies in the band of from 30 c./ s. to 50,000

c./s., modulating the carrier frequency which has a frequency of about 100 mc./s., with a sweep of 75 kc./s., in frequency.

The stereophonic receiver is provided with an antenna 1 and an intermediate-frequency stage 2 comprising a mixer stage and an oscillator 3 connected thereto, while the intermediate-frequency oscillations which are obtained from this oscillator by mixing and lie in the band of 10,700 kc./s., are supplied, after amplification and, if required, limitation in the intermediate-frequency stage 2, to an output band-pass filter comprising two coupled circuits 4 and 5.

The band-pass filter 4 and 5 forms part of a frequency detector of a type known for the detection of normal frequency-modulation transmissions which contains two rectifiers 6 and 7 which are connected, with opposite conductivity with respect to each other, between the ends of the circuit 5 and an output impedance 8, while a centre tapping on the circuit 5 is connected to the end of a coupling coil 9 coupled to the circuit coil 4. The output impedance 8 of the frequency detector comprises a resistor 10 shunted by the series combination of two capacitors 11, 12, of which the junction is earthed, and by a smoothing capacitor 13, the output voltage of the frequency detector being taken from a centre tapping on the output resistor 10.

Thus, at the output impedance of the frequency detector an output voltage appears which is produced by the sum signal A+B in the band of from 30 c./s. to 15,000 c./s. and by the diiference signal AB, which modulates the sub-carrier frequency in amplitude and lies in the frequency band of from 20 kc./s. to 50 kc./s., and this output voltage is supplied, through a blocking capacitor 14, to a. device 16, which will be described hereinafter, in order to recover the coherent stereophonic signals A and B, which are supplied, after low-frequency amplification in separate low-frequency amplifiers 17 and 18, to reproducing devices :19 and 20.

The low-frequency amplifiers 17 and 18, which are designed similarly, are constituted by triodes provided with grid leak resistors 21, 22 and not-shunted cathode resistors 23, 24 and 2-5, 26 which provide negative feed-back of the triodes 17, 13'. The amplified coherent stereophonic signals A and -B are taken from output resistors 27 and 28 which are included in the anode circuits of the triode 17 and 18 and are connected, as is shown diagrammatically in the figure, to the reproducing devices 19 and 20- through blocking capacitors 29 and 30.

In order to enable the coherent stereophonic signals A and B to be recovered from the output voltage of the frequency detector in a simple manner with a high quality of reproduction, this output voltage, which comprises the sum signal A+B and the difference signal AB, which modulates the sub-carrier frequency in amplitude, is supplied to the device 16 which is provided with two amplitude detection devices 31 and 32 of opposite polarities, the input circuits of these devices being connected in parallel with one another to the output circuit of the frequency detector 6, 7. These amplitude detection d vices are each provided with a rectifier cell 31 and 32 respectively and with the output impedance connected thereto which comprises the parallel combination of a resistor 33 and 34 respectively and a capacitor 35 and 36 respectively, the output voltages of the amplitude detection devices 31 and 32 being supplied, through blocking capacitors 37, 38 respectively, to the control grids of triodes 17 and 18 respectively, which are connected as low-frequency amplifiers. With the exception of the opposite conductivity directions of the rectifier cells 31 and 32, the amplitude detection devices 31 and 32 are designed similarly.

If at the input of the described detection devices 31 and 32 there appears the output voltage of the frequency detector 6, 7, which voltage comprises the audio-frequency sum signal A-f-B and the dilference signal A-B which modulates the sub-carrier frequency in amplitude, both amplitude detection devices 31 and 32 effect an amplitude detection of the difference signal AB modulating the sub-carrier frequency, while the audio-frequency sum signal A-l-B acts as a threshold voltage for the two rectifier cells 31 and 32 which are connected with asymmetric conductivity with respect to each other, so that the operating points of the two rectifier cells 31 and 32 are shifted in opposite directions in accordance with the sum signal A-l-B. Thus, there is produced not only an amplitude detection of the envelope signal constituted by the difference signal A--B, but also addition and subtraction of the audio-frequency difference and sum signals A-B and A-l-B, with the result that at the output circuits 33, 35 and 34, 36 respectively of the amplitude detection devices 31 and 32, precisely the coherent stereophonic signals A and B are produced which, through the two lowfrequency amplifiers 17 and 18 respectively, are supplied to the two reproducing devices 19 and 20. While the structure of the stereophonic receiver is very simple, frequency-dependent phase-shifts, which may occur in the recovery of the coherent stereophonic signals A and B from the output voltage of the frequency detector, are materially reduced by the absence of filters, and this is of particular advantage for a stereophonic reproduction of high quality.

In order to improve the quality of reproduction, it is useful to connect, between the frequency detector 6, 7 and the amplitude detection devices 31 and 32 connected with opposite conductivity with respect to each other, a series resistor 39 which is at least equal to from A to /3 of the discharge resistors 33 and 34 of the detection capacitors 35 and 36, respectively, for it is found that due to the obtained increase in the charge time constant of the detection capacitors 35 and 36 relative to the discharge time constant of these capacitors 35 and 36, in the detection of the sub-carrier frequency, which is comparatively low as compared with the frequency of the voltage modulating it, detection distortions are reduced. If required, this object may alternatively be achieved by connecting a series resistor in series with the output impedances 33, 35 and 34, 36 of each of the amplitude detection devices 31 and 32.

In addition to a high quality of reproduction and a fixed phase relationship, it is found that in the stereophonic receiver described cross-talk between the two channels, which may be due to mutual couplings or to differences in level between the A+B signal and the A-B signal, can be reduced to less than from 30 to 40 db through the entire audio-frequency band of 1S kc./s., for it was ascertained that the cross-talk voltages have a fixed phase relationship with respect to the stereophonic signals A and B, in particular, these cross-talk voltages are substantially in phase with or in phase opposition to the stereophonic signals A and B. In the stereophonic receiver described, an effective cross-talk reduction is achieved in a simple manner by the use of a method of compensation which consists in that an attenuator providing a suitable degree of attenuation is connected to a point of each of the reception channels, which attenuator is fed with a signal voltage which is derived from the other channel and is in phase opposition to the cross-talk voltage concerned.

In particular, it is found that the voltages in the two channels can be represented by:

AiaB Bios/1 where a3 and 11A are the cross-talk voltages and a the cross-talk factors, which are identical in value and in phase so that the cross-talk compensation can be further simplified. As will be seen from the above formulas, the

two cross-talk factors a can have a positive or a negative value.

If the cross-talk factors on show negative polarity, so that the signals in the two channels are represented by A-uB and B-aA, in the embodiment shown cross-talk is compensated for by the connection, between the two output impedances 33, 35 and 34, 36 respectively of the amplitude detection devices 31 and 32 respectively, of a connecting resistor 40 which is shunted by a capacitor 15, the time constant of this resistor 40* being approximately equal to the time constant of the output impedances 33, 35 and 34, 36 respectively of the amplitude-detection devices 31 and 32 respectively forms a frequency-independent voltage divider, so that through the network 44 15 a fraction of the output voltage of the amplitude detector 32 is supplied to the output circuit 33, 35 of the amplitude detector 31 and simultaneously a same fraction of the output voltage of the amplitude detector 31 is supplied to the output circuit 34, 36 of the amplitude detector 32. If the network 40, 15 is adjusted so that the voltage division ratio is exactly equal to the cross-talk factor a, a complete cross-talk compensation is obtained, for in this event the compensation voltage supplied through the network 40, 15, which voltage is in phase opposition to the cross-talk voltage, is exactly equal in value to this cross-talk voltage. Experiments have shown that satisfactory results are obtained, when the capacitor 15 is omitted from the compensating network 40, 15.

With a positive value of the cross-talk factor 0:, so that the signals in the two channels are represented by A-l-uB and B-l-aA, the compensation is effected similarly in that a'connecting resistor 41 is connected between the cathode resistors 23, 24 and 25, 26 of the amplifier valves 17 and 18. In the practical embodiment, the grid leak resistors 21 and 22 of the amplifier valves 17 and 18 respectively are connected to tappings on the cathode resistors 23, 24 and 25, 26 respectively, so that larger cathode resistors 23, 24 and 25, 26 respectively can be used, and this is of advantage for practical reasons.

When the network 40, :15 and the resistor 41 are both used, an effective compensation of the cross-talk can be achieved in a simple manner for all values of the crosstalk factors which may be found, as has been explained hereinbefore. However, for the adjustment of this compensation, only a single of these compensating networks 40, 15 and 41 need be variable, for if the network 40, 15 of the resistor 41 is adjusted to a fixed value such that the cross-talk factors always have a certain polarity, this cross-talk can always be compensated for by suitable adjustment of the other compensating network 41 or 40, 15. For this purpose, preferably the network 40, 15 connected between the output impedances 33, 35 and 34, 36 is fixed while the resistor 41 is variable, as is shown in FIG. 1.

To sum up, it is found that the stereophonic receiver described has all the features required for excellent stereophonic reproduction, namely a high quality of reproduction, a fixed phase relationship and cross-talk reduced to less than 30 to 40 db, which is amply sufficient for an excellent stereophonic reproduction. The design of the stereophonic receiver is simple and its structure completely symmetrical, so that its cost can be materially reduced, for example, the amplifier valves 17, 18 can be designed as one double valve; the additional cost of the stereophonic receiver substantially amounts to the cost of an additional loudspeaker. Furthermore, owing to its symmetrical character this stereophonic receiver is suitable to be used for stereophonic gramophone record reproduction while it can also be adapted to normal frequency-modulation reception by interconnecting the output circuits 33, 35 and 34, 36 of the two amplitude detection devices 31 and 32 by means of a switch 42.

The following are details of an apparatus which was extensively tested in practice;

Diodes 31, 32 2OA79.

Resistor 39 47 KS2.

Resistors 33, 34 100 KS2. Capacitors 35, 36 1000 ,uaf. Capacitor 4'70 ,u tf. Resistor 40 10G KS2. Resistors 23, 25 1.8 KS2. Resistors 24, 26 82 K9.

Resistor 41 100 KS2 variable. Valves 17, 18 ECC83.

Finally, it should be noted that the use of the connecting resistor 40 connected between the output impedances 33, 3'5 and 34, 36 provides additional advantages, for through this connecting resistor 40' a fraction of the signal voltage set up at the output impedance 33, 35 is supplied to the output impedance 33, 36 and conversely, so that there is produced at each of these output impedances 33, 35 and 34, 36 a signal component ,8(A;IB) which is in phase with the audio-frequency sum signal A-l-B supplied to the amplitude detection devices 31 and 32, with the result that the shifting of the operating points of the diodes 31 and 32 by this sum signal A+B is reduced, and this is of importance for a further improvement of the quality of reproduction.

FIG. 2 shows a transistorized stereophonic receiver of the type described. Similar elements are designated by like reference numerals.

Instead of a valve amplifier, in the stereophonic receiver shown in this figure use is made of a transistor amplitier provided with two transistors 44 and 45 in common emitter arrangement. Collector resistors 46 and 47 are connected in the collector circuits of the transistors 44 and 45, respectively, the reproducing devices 19 and being connected to these collector resistors through blocking capacitors 48 and 49 respectively, while the collectors are connected through resistors 50 and 51 respec tively to the associated bases. Emitter resistors 52 and 53 are connected in the emitter circuits of the transistors 44 and 45 respectively, and connected together through a variable resistor 54 in order to provide cross-talk compensation. In the manner described with reference to FIG. 1, the coherent stereophonic signals A and B are reproduced by the reproducing devices 19 and 20.

Experiments have shown that the stability of the transistorized stereophonic receiver described is sutficient for stereophonic broadcast reception.

The following are data of the stereophonic broadcast receiver described:

Transistors 44, 45 0071 Resistors 46, 47 "KS2" 5.6 Resistors 52, 53 "KS2" 3.9 Resistors 5t}, 51 "K9" 47 Resistor 54 Kt2 5 In order to recover the coherent stereophonic signals A and B, in the embodiments shown in FIGS. 1 and 2, the sum signal (A-|-B) is supplied, together and in series with the difference signal (AB) modulating the subcarrier frequency, to the amplitude detection devices 31, 32 connected with opposite conductivity to one another, however, for this purpose the sum signal A+B may alternatively be supplied in phase opposition to the two amplitude detection devices, in which event the amplitude detection devices must be connected with equal conductivity. For this purpose, a frequency detector may be used the output circuit of which is connected in pushpull with respect to earth, either of the pushpull output voltages being supplied to one of the equally conductive amplitude detection devices. It is a feature of all these embodiments that the two amplitude detection devices to which the audio-frequency sum signal A -i-B and the difference signal AB, which modulates the sub-carrier frequency in amplitude, are supplied together, are 'connected with opposite conductivity with respect to the audio-frequency sum signal A+B. Tests have shown that the embodiments according to FIGS. 1 and 2 are to be preferred, inter alia by reason of their high quality of reproduction and the simplicity of their design.

Finally, it should be noted that the simplicity of the stereophonic reproducing arrangement described renders it very suitable for use for magnetic tape reproduction, for the audio-frequency sum signal A-i-B and the difference signal AB modulating the sub-carrier frequency can be recorded in one track by means of a single recording head, so that no radical alterations need be made in the existing magnetic tape recording apparatus.

What is claimed is:

l. A circuit for the stereophonic reproduction of signals of the type having a sum signal of first and second coherent stereophonic signals, and a difference signal of said first and second signals, said difference signal being in the form of amplitude modulation of a subcarrier, said circuit comprising first and second amplitude detector circuits comprising first and second unidirectional current devices respectively, means applying said sum and difference signals to said first and second amplitude detector circuits, said first and second unidirectional current devices being connected with opposite polarities with respect to said sum signal, and means connected to said first and second detector circuits to derive first and second stereophonic output signals respectively.

2. A circuit for the stereophonic reproduction of signals of the type having a sum signal of first and second coherent stereophonic signals, and a diiference signal of said first and second signals, said difference signal being in the form of amplitude modulation of a subcarrier, said circuit comprising means for providing said signals, first and second amplitude detector circuits having first and second unidirectional current devices respectively for detecting said difference signal and first and second impedance means connected to provide a bias for said first and second unidirectional current devices respectively, means applying said sum and difference signals to said first and second amplitude detector circuits, said applying means being connected to provide opposite conductivity of said unidirectional current devices with respect to said sum signal whereby opposite biases are provided by said sum signal on said unidirectional current devices, and means connected to said first and second detector circuits to derive first and second stereophonic output signals respectively.

3. A circuit for the stereophonic reproduction of signals of the type having a sum signal of first and second coherent stereophonic signals, and a diiference signal of said first and second signals, said difference signal being in the form of amplitude modulation of a subcarrier, said circuit comprising means for providing said signals, first and second series circuits of first and second unidirectional current devices and first and second impedance means respectively, means applying said sum and difference signals to each of said series circuits for opposite current flow with respect to said sum signal, and means connected to said first and second series circuits for providing first and second output signals respectively.

4. A circuit for the stereophonic reproduction of signals of the type having a sum signal of first and second coherent stereophonic signals, and a difference signal of said first and second signals, said difference signal being in the form of amplitude modulation of a subcarrier, said circuit comprising means for providing said signals, first and second terminals, a first series circuit of a unidirectional current device and a first impedance connected in that order between said first and second terminals, a second series circuit of a second unidirectional current device and a second impedance connected in that order between said first and second terminals, said unidirectional current devices being connected for opposite conductivity with respect to said terminals, means applying said sum and difference signals between said terminals, and means deriving first and second stereophonic output signals at the junctions of said first impedance and first unidirectional current device, and the junction of said second impedance and second unidirectional current device, respectively.

5. The circuit of claim 4, comprising first and second electron discharge devices having grids and cathodes, means applying said first and second stereophonic output signals to the grids of said first and second discharge devices respectively, unshunted first and second resistors connected to the cathodes of said first and second discharge devices respectively, and resistor means connected between said first and second resistors.

6. The circuit of claim 4, comprising first and second transistors having base, emitter, and collector electrodes, said transistors being connected in common emitter arrangement With unshunted emitter resistors, means applying said first and second output signals to the bases of said first and second transistors respectively, resistor means connected between the unshunted emitter resistors of said transistors, and first and second output circuits connected to the collectors of said first and second transistors respectively.

7. The circuit of claim 4, comprising a crosstalk reducing resistor connected between the junction of said first impedance and first unidirectional current device, and the junction of said second impedance and said second unidirectional current device.

8. The circuit of claim 7, comprising a capacitor connected in parallel with said crosstalk reducing resistor, the time constant of said resistor and capacitor being substantially equal to the time constant of said first and second impedances.

9. A circuit for the stereophonic reproduction of signals of the type having a sum signal of first and second coherent stereophonic signals, and a difference signal of said first and second signals, said difference signal being in the form of amplitude modulation of a subcarrier, said circuit comprising means for providing said signals, first and second terminals, means applying said sum and difference signals between said terminals, a series resistor having one end connected to said first ter minal, first and second unidirectional current devices having opposite electrodes connected to the other end of said series resistor, first and second output resistors connected between the other ends of said first and second unidirectional current devices and said second terminal respectivly, first and scond capacitors connected in parallel with said first and second resistors respectively, and first and second output terminals connected to the junctions of said first and second resistors and said first and second unidirectional current devices respectively,

said series resistor having a value from A to /3 of the value of said first and second resistors.

10. A circuit for the stereophonic reproduction of signals of the type having a sum signal of first and second coherent stereophonic signals, and a difference signal of said first and second signals, said difference signal being in the form of amplitude modulation of a sub-car-rier, said circuit comprising means for providing said signals, first and second terminals, a series resistor having one end connected to said first terminal, a first series circuit of a first unidirectional current device and first resistor connected in that order between the other end of said series resistor and said second terminal, a second series circuit of a second unidirectional current device and a second resistor connected in that order between the other end of said series resistor and said second terminal, first and second capacitors connected in parallel with said first and second resistors respectively, means applying said sum and difference signals between said first and second terminals, said unidirectional current devices being connected for opposite conductivity with respect to said terminals, first and second amplifying devices having input electrodes, common electrodes, and output electrodes, means connecting the junction of said first unidirectional current device and first resistor to the input electrode of said first amplifying device, means connecting the junction of said second unidirectional current device and second resistor to the input electrode of said second amplifying device, third and fourth unshunted resistors connected to the cathodes of said first and second amplifying devices respectively, and means connected to the output electrodes of said first and second amplifying devices for deriving first and second stereophonic signals respectively.

11. The circuit of claim 10, comprising resistor means connected between said third and fourth resistors.

12. The circuit of claim 10, comprising means for selectively interconnecting the junction of said first unidirectional current device and first resistor and the junction of said second unidirectional current device and second resistor for reception of non-stereophonic signals.

13. The circuit of claim 10, comprising a crosstalk reducing resistor connected between the junction of said first unidirectional current device and said first resistor and the junction of said second unidirectional current device and said second resistor.

14. The circuit of claim 13, in which said crosstalk reducing resistor has a fixed value, comprising variable resistor means connected between said third and fourth resistors, whereby variation of said variable resistor provides correction of positive and negative crosstalk factors.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A CIRCUIT FOR THE STEREOPHONIC REPRODUCTION OF SIGNALS OF THE TYPE HAVING A SUM SIGNAL OF FIRST AND SECOND COHERENT STEREOPHONIC SIGNALS, AND A DIFFERENCE SIGNAL OF SAID FIRST AND SECOND SIGNALS, SAID DIFFERENCE SIGNAL BEING IN THE FORM OF AMPLITUDE MODULATION OF A SUBCARRIER, SAID CIRCUIT COMPRISING FIRST AND SECOND AMPLITUDE DETECTOR CIRCUITS COMPRISING FIRST AND SECOND UNIDIRECTIONAL CURRENT DEVICES RESPECTIVELY, MEANS APPLYING SAID SUM AND DIFFERENCE SIGNALS TO SAID FIRST AND SECOND AMPLITUDE DETECTOR CIRCUITS, SAID FIRST AND SECOND UNIDIRECTIONAL CURRENT DEVICES BEING CONNECTED WITH OPPOSITE POLARITIES WITH RESPECT TO SAID SUM SIGNAL, AND MEANS CONNECTED TO SAID FIRST AND SECOND DETECTOR CIRCUITS TO DERIVE FIRST AND SECOND STEREOPHONIC OUTPUT SIGNALS RESPECTIVELY. 